Eanes W F, Kirchner M, Yoon J
Department of Ecology and Evolution, State University of New York, Stony Brook 11794.
Proc Natl Acad Sci U S A. 1993 Aug 15;90(16):7475-9. doi: 10.1073/pnas.90.16.7475.
Proponents of the neutral theory argue that evolution at the molecular level largely reflects a process of random genetic drift of neutral mutations. Under this theory, levels of interspecific divergence and intraspecific polymorphism are expected to be correlated across classes of nucleotide or amino acid sequences with different degrees of functional constraint, such as synonymous and replacement sites. Nucleotide sites with reduced polymorphism should show comparably reduced levels of interspecific divergence. To examine this hypothesis, we have sequenced 32 and 12 copies of the glucose-6-phosphate dehydrogenase (G6pd) gene in Drosophila melanogaster and Drosophila simulans, respectively. Both species exhibit similar levels of nucleotide polymorphism at synonymous sites. D. melanogaster shows two amino acid polymorphisms, one associated with the cosmopolitan allozyme polymorphism and a second with an allozyme polymorphism endemic to European and North African populations. In contrast, D. simulans shows no replacement polymorphism. While synonymous divergence between species is 10%, which is typical of other genes, there are 21 replacement differences. This level of amino acid sequence divergence, when contrasted with levels of amino acid polymorphism, silent polymorphism, and divergence, is in 10-fold excess over that expected under the neutral model of molecular evolution. We propose that this excess divergence reflects episodes of natural selection on G6pd resulting in fixation of advantageous amino acid mutations in these two recently separated lineages.
中性理论的支持者认为，分子水平的进化在很大程度上反映了中性突变的随机遗传漂变过程。根据这一理论，在具有不同功能限制程度的核苷酸或氨基酸序列类别中，种间分歧水平和种内多态性水平预计会相互关联，例如同义位点和替换位点。多态性降低的核苷酸位点应显示出相应降低的种间分歧水平。为了检验这一假设，我们分别对黑腹果蝇和拟果蝇的32个和12个葡萄糖-6-磷酸脱氢酶（G6pd）基因拷贝进行了测序。两个物种在同义位点表现出相似的核苷酸多态性水平。黑腹果蝇显示出两种氨基酸多态性，一种与世界性的等位酶多态性相关，另一种与欧洲和北非种群特有的等位酶多态性相关。相比之下，拟果蝇没有替换多态性。虽然物种间的同义分歧为10%，这是其他基因的典型情况，但有21个替换差异。与氨基酸多态性水平、沉默多态性水平和分歧水平相比，这种氨基酸序列分歧水平比分子进化中性模型预期的高出10倍。我们认为，这种过度分歧反映了G6pd上的自然选择事件，导致这两个最近分化的谱系中有利氨基酸突变的固定。
